Generally, a multilayer printed wiring board is composed of a plurality of copper-plated plates laminated and pressed, and the layers are electrically connected therebetween to form a multilayer printed wiring board. Such interlayer connection is formed in such a manner that a small hole is formed at a predetermined portion of the multilayer printed board by use of a drill so that the small hole is plated (through-hole-plated) at the internal surface thereof. Each layer at the multilayer printed board comprises copper foil and an epoxy resin impregnated base material setting layer. When the small hole is formed by use of an old drill, the epoxy resin of the epoxy resin impregnated base metal setting layer melts by the frictional heat and covers the copper foil part appearing at the internal surface of the hole, the covered portion being called the smear. Therefore, even when a plated layer is formed at the internal surface of the small hole, the smeared part causes a poor electrical connection between the plated layer and the copper foil, resulting in poor conduction. Also, in a case where the old drill is used to form a small hole in the multilayer printed board, as above-mentioned, the internal surface of the small hole becomes rough so that air foam becomes attached to the rough surface at the time of the plating to cause disconnecting of a portion at the plated layer, resulting in poor conduction.
In a case where the small hole is formed in the multilayer printed board, when the smear is produced at the internal surface of the small hole or the internal surface becomes rough, the plated layer of through-hole plating is poor in conduction, whereby prior to the through-hole plating, the internal surface condition of the small hole has been observed. When a hole diameter is larger or the board is small in thickness, the internal surface can be observed obliquely by use of a microscope. However, when the hole diameter is very small as 0.3 to 4 mm, it is impossible to observe the internal surface reflected by a small mirror inserted into the hole or to observe obliquely the entire surface. Hence, an object to be inspected is sampled for inspection and subjected to a breakdown test to thereby observe the internal surface of the hole. In detail, the multilayer printed board to be inspected is sampled and cut by intersecting .the hole so that the internal surface is visually inspected or observed by an electron micrograph. Such observation, however, takes much time and breaks down the object to be inspected, thereby being unreasonable.